Dear Sir, what you may not have taken into consideration is the CRUSHING strength that PVC does not have.
Plastic although OK for certain applications is not designed for any load bearing weight.
Before you waste you time and money why not do some research regarding the compacting limitations of the materials your using.
You may decide to use Cast Iron if soil conditions warrant Or possibly schedule 80 plastic.
You really do not want a material that will float up like a cork either.
Rather then help you design a system you should try to do just a little homework yourself.
If you have any questions please feel free to contact me E mail.
By the way I installed piping under a creek that was 20 feet deep and fast flowing so nothing is impossible look at the Tunnels JUST take you time to reason it out.
This could be a fun project for a do it yourself person.